In order to meet the ever-increasing demand for transmission bandwidth in communication networks, the development of techniques for Wavelength Division Multiplexing (WDM) is increasing in importance. In particular, in dense WDM (DWDM), the use of the available optical bandwidth is optimized by increasing the system spectral efficiency (i.e., the ratio between signal bandwidth and channel spacing). This is achieved by employing many closely spaced carrier wavelengths multiplexed together onto a single waveguide such as an optical fiber and/or by increasing the signal modulation speed of every single data channel.
Because the channels' spectra are more closely packed, the ability of separating or merging channels without introducing any additional signal penalty is becoming of great importance. Therefore, optical filters are being developed with filter response and frequency alignment that provide for a better match to the signal frequencies.
An important class of such optical filters is represented by Arrayed Waveguide Gratings (AWGs). An arrayed waveguide grating (AWG) is a planar structure comprising a number of array waveguides whose arrangement emulates the functionality of a diffraction grating (see e.g. M. K Smit and C. van Dam, “Phasar based WDM devices: Principles, design, and applications,” IEEE J. Select. Topics Quantum Electron., vol 2, pp.236-250, 1996). AWGs are commonly used as multiplexers or demultiplexers (i.e., devices that can merge a multitude of optical frequencies from multiple inputs to a single output port or separate a multitude of optical frequencies from a single input to multiple output ports, respectively). Furthermore, AWGs can also be designed to perform as passive, wavelength selective, strictly non-blocking cross-connects for sets of optical channels. For example, AWGs can simultaneously operate as a multiplexer and demultiplexer by distributing and recombining multiple frequencies entering any of a multitude of input ports into any of a multitude of output ports (see e.g. C. Dragone, “An N×N optical multiplexer using a planar arrangement of two star couplers,” IEEE Photon. Technol. Lett., vol. 3, pp. 812-815, 1991). Because of this property and uncommon versatility, AWGs are attracting an increasing interest for large optical cross-connect systems.
The use of AWGs, however, presents some limitations. Due to the intrinsic diffraction characteristics of an AWG, the maximum channel count or maximum spectral width of this kind of devices may be limited. Additional limitations arise if the AWG is designed to cross-connect channels that are equally spaced in frequency to be compliant with the industry International Telecommunications Union (ITU) standard frequency grid as opposed to being equally spaced in wavelength (see e.g. P. Bernasconi, C. Doerr, C. Dragone, M. Cappuzzo, E. Laskpwski, and A. Paunescu, “Large N×N Waveguide grating routers,” J. Lightwave Technol., vol. 18, pp. 985-991, 2000).